Degenerative expander-compressor circuit



Jan. 5, i943. E. M. SORENSEN 2,307,308

DEGENERATIVE EXPANDER-COMPRESSOR CIRCUIT 7 Filed Aug. 2, 1940 2 Sheets-Sheet 2 ATTORNEYS Patcnted Jan. 5, 1943 UNITED STATE DEGENERATIVE EXPANDER-OOMPRESSOR CIRCUIT Edward M. Sorcnsen, Dayton, Ohio, assignor to The Soren Corp., Dayton, Ohio, a corporation of Delaware Application August 2, 1940, Serial No. 349,863

6 Claims.

. This invention relates broadly to degenerative circuits, and in particular to circuits for providing expansion or compression of an alternating current voltage in a non-linear manner. Compres'sion circuits are quite frequently used in connection with radio broadcasting and phonograph recording, and expansion circuits are useful in connection with amplifiers or phonograph reproducers for restoring the audio tone to its original quality before it is compressed in the broadcasting station apparatus, or in the phonograph recording apparatus.

Another application of the compression circuit which finds a greatmumber of uses is for limiting. the audio tone to a sharp cut-ofi at a given decibel quantity. An illustration of this use is as a volume-limiting control for a radio loud speaker.

Circuits heretofore used for compression and expansion have been subject, among other disadvantages, to anundue amount of distortion,

and also have not been capable of controlling the expansion'or compression over a wide range of -.input signal voltage and/or frequency variation. It is therefore an object of my invention to provide a novel degenerative circuit which will provide compression over a wide range of input signal variation, and a wide range of expansion, while limiting distortion to an exceedingly small amount. With my novel circuit, expansion and compression can be provided to an extent which is limited only by the gain of the amplifier without distortion within itself.

It is another object of my invention to provide degeneration control through the use of an element whose resistance progressively decreases with an increase in applied voltage. Such resistance may be termed a negative resistance and can be obtained in several ways, as'for instance, by "th'use of a. vacuum tube with its grid and plate circuit connected to a common voltage sourc'e; by the use of a material which rectifies, such as galena, pyrite, carborundum or copper oxide. importance in this connection is thyrite, disclosed in Patent No. 1,822,742 issued to Karl B. McEachron on September 8, 1931. The! resistance of this material changes as a hyperbolic function with respect to the amount of voltage applied.

Many other objects, advantages and applications of my invention" will be apparent from the following description and the accompanying drawings.

In the drawings:

Figure 1 shows a basic circuit, illustrating my invention as used for compression. 1

Figure 2 is a part of the circuit shown in Fig-\ ure 1, as rearranged to provide expansion.

Figure 3 is a combined expansion and compression circuit with a double-throw switch shown in position for providing compression, and in dotted position for providing expansion, and further differing from Figure 1 in the circuit connections to the amplifier.

Figure 4 is a basic circuit illustrating the use of a negative resistance material, such as One material of exceptional thyrite, for compression.

Figure 5 is a circuit similar to Figure 4, but arranged for expansion. 1

It will be seen from the drawings that the invention, in principle, contemplates the connection of a circuit between two points in an amplifying circuit, which points have a phase diiierence of approximately 180 degrees, and providing in said circuit a resistance which decreases at a greater rate than the voltage increases. Such resistance is provided in one variation by means of a vacuum tube having a/grid controlled bysome of the rectified audio voltage. In this way the resistance of the vacuum tube is varied by changing the bias on the grid, and the bias on the grid is proportioned to the increase or decrease in the audio signal. By changing the polarity of the rectifier, the bias on the grid can be made more positive as the audio signal increases, thereby enabling a greater amount of degeneration to provide compression, or the bias on the grid can be made more negative as the audio signal increases, decreasing the amount of degeneration to thereby provide expansion of the audio signal. In lieu of the vacuum tube circuit, one of the negative resistance materials discussed above may be used.

It will be understood that the connection of the circuit to the amplifier at points of opposite voltage phase maybe made by connecting across the plate and grid of a vacuum tube, or across any odd number of amplification stages. Also, that a transformer reverses the voltage phase and must be counted as a stage of amplification for this purpose.

Referring to the drawings, Figure 1 shows an elementary form of an application of the invention. In the drawings there is shown schematically an audio amplifier 10 which may comprise one or more stages of amplification, and as illustrated, shows a two-stage amplification by means r or the tubes II and I2. The alternating voltage input between the points 13 and I4 is coupled to the grid l5 by means of the transformer I. The voltage between the points Is and I1 is 180 degrees out of phase with the input signal, due

to the well known action of the transformer. The audio signal is successively passed through the amplifier until it is present in the primary winding It at the output of the amplifier. The secondary winding I9 is coupled to the primary winding, and provides the output signal at points 2| and 2| for operating the loud speaker or other apparatus. A secondary winding 22 is also coupled to the primary I8 and connects to the plate 23 of the diode rectifier 24. The voltage of the alternating current, in passing through the audio amplifier to the secondary winding 22, has been successively reversed until the voltage between the winding 22 and ground I1 is 180 degrees out of phase with the voltage between the grid I 5 and ground II. The signal on the secondary 22 is also supplied to the cathode 25 of tube 26. Tube 28 functions as a variable resistance and not as an amplifying tube. There is no phase reversal of the voltage in passing throughthis tube. The plate 21 is supplied with current from potential source 28. It will thus be seen that there is a direct current circuit from potential 28, through tube 26. tube 24 to ground I1. There is also a pulsating direct current from the secondary 22, through tube 26 to the secondary of the transformer IE, but this current is 180 degrees out of phase with the voltage on the secondary of transformer I6.

Grid 29 of tube 26 is Provided with a steady negative bias by battery 30, the biasing current flowing through the winding 22 to ground. There is further impressed on the grid a rectified pulsating voltage from rectifier 24. It will therefore be seen that on the plus cycles of the alternating current in the circuit from winding 22 to the secondary of the transformer I6, that the bias on grid 29 will be made less negative and the flow of current through the tube will be facilitated, and also that as the voltage present on secondary 22 increases, an increased voltage of opposite phase relation will be impressed upon the secondary of transformer l6, thereby cancelling out a quantity of the signal voltage being applied across points l3 and I4. In this way the circuit serves to maintain approximately constant the output of the amplifier, regardless of variation in the input signal voltage. Variable resistor 35, connected in the cathode circuit of diode tube 24, is used to provide adjustment for the amount of compression which is desired to be obtained, which follows as the amount of voltage being applied to grid 29 is controlled thereby. Condenser 36 serves as a filter condenser to filter out any alternating current component from the voltage being applied to the grid 29.

The foregoing circuit provides a control for single-sided amplifiers. The circuit can be adapted to function on push-pull arrangements by the addition of duplicate equipment.

The same circuit, with a few modifications, is used to expand the audio signal being applied to points l3 and 14. Such a modified circuit is 1 shown in Figure 2. A and B designate the points in Figure 1 where the circuit of Figure 2 is connected in Figure 1. It will be seen that the only difference is that the cathode and anode connections of diode rectifier tube 24 are reversed. Since the diode tube is now reversed, it provides a negative polarity connection to grid 29 and functions to increase the negative bias applied to the grid 29 as the voltage on secondary winding 22 increases. In this manner, the amount of voltage which is normally being fed back to the secondary winding of transformer I. out or phase with the voltage present thereon from the transformer is reduced as the signal voltage increases, thereby enabling expansion of the received signal.

The circuit shown in Figure 3 is a combination arrangement of circuits for providing either expansion or compression. The circuit further differs from the circuit described in F gure 1. among other things, in that a part of the input signal voltage is rectified and employed to control the grid of the tube in the circuit connecting the output voltage back to the input side of the second stage of the amplifier, in order to control the expansion or compression.

Referring to the drawings, the input terminals 50 and 5| are connected through variable resistor 52, which serves as the volume control to the grid 53 of vacuum tube 54 constituting the first stage of the amplifier. The plate 55 of tube 54 is connected through condenser 58 to the grid 51 of the tube 58, constituting the second stage of the amplifier. Battery terminals 59 and 60 provide the plate current for tubes 54 and 58 and tube 6|, which corresponds in this circuit to tube 26 in Figure 1. The output voltage is taken off at terminals 62 and 63. Rectifying tube 64 is connected to input terminal 50 through transformer 65, the latter serving to isolate the voltage in the circuit of tube 64 above ground. Then, by means of the condensers 68 and 61 connected across the plate and cathode circuits of tube 64 with a ground connection 88 intermediate the condensers 66 and 6.1, voltage at terminal 69 connected to the cathode of vacuum tube 84 is made positive with respect to ground, and voltage at terminal 10 connected to the plate circuit of vacuum tube 64 is negative with respect to ground. By means of a three-pole, doublethrow switch H the positive terminal 69 is connected through conductor 12, arm I: and conductor I4 to the grid 15 of tube 6|,in the fullline position shown. In the dotted-line position In the full-line position of switch 1|, negativeterminal 10 is connected to the cathode 8|, through arm 16 and suitable conductors; and in the dotted-line position, plus terminal 69 is connected to the cathode 8|, through arm 82 and a suitable conductor network. Condenser 80, connected across the grid 15 and cathode 8| of tube 6|, serves as a filter condenser in the same manner as condenser 36 in Figure 1. Variable resistor 83 across battery 18 enables adjustment of the fixed negative bias.

It will thus be seen that in operation there is a direct current flowing from battery terminal 60, through tube 6 I, variable resistor 84 connected to the cathode of tube 6|, and back to the negative battery terminal 59. There is also the alternating current of the amplified signal voltage flowing through the tube 5| from output terminal 62, through a portion of the variable resistor 84 to the circuit of grid 51 of the second amplifying tube. In this connection, it should be noted that the signal voltage on grid 51 is degrees out of phase with the input signal voltage being applied to grid 53, and that again the output voltage in the plate circuit of tube 58, which is commonly connected to the plate circuit of tube 6|, is 180 degrees out of phase with the signal voltage being applied to grid 51'. There is no change in tube It will therefore be seen that the alternating current voltage passing through tube 9| to grid 51 is 180 degrees out of the three-pole switch II is thrown to the dotted position, putting bias battery 18 and the negative J thyrite" being in shunt connection with the in-' terminal of tube 64 in the grid circuit of tube 61, the grid is made more negative, thereby reducing the amount of alternating current flow-, ing through tube 6!, and as the output voltage of tube 58 tends to increase the amount of voltage being fed to the grid 51 of tube 58 from tube BI, and out of phase with the voltage on grid 51 from tube 54, is decreased, thereby providing the desired expansion.

Variable resistor 83 across battery I8 serves to set the threshold level for expansion. By the term "threshold level is meant that initial point at which .the expansion is made operative. Variable resistor 94 corresponds to variable resistor 35 in Figure 1, and-may be adjusted to control the amount of degeneration being applied to grid 51 of tube 59. For expansion a greater amount of degeneration will be initially applied to the grid, so that it may be reduced as the audio signal increases, and for compression a less amount of degeneration will be applied initially,'so that the tube 6! may supply more as the signal increases. I

In Figure 4 is shown a basic amplifying circuit having input terminals 99 and 9i, output terminals 92 and 93 having a resistor 99 between them, and a vacuum tube 95 arranged asan amplifier. Connected across plate 99 and grid 91 of the vacuum tube is a circuit 98 having a condenser 99 in series with a piece of thyrite Hill as a non-linear or negative resistance. Because of the special property of thyrite to conduct a current which increases in value out of proportion to an increase in voltage applied, it will be seen that as the current in plate 96 increases. a current increases in circuit 99 out of proportion to the plate circuit current. Since the current in plate circuit 99 is 180 degrees out of phase with that of the input signal on grid ,91, the current conducted by circuit 99 opposes the incoming signal in a manner corresponding to the amplification factor of tube 95, thereby directly between the plate circuit of one stage and the plate circuit of the preceding stage. It

may also be connected in a push-pull amplifier circuit in the samemanner.

In Figure 5 is shown an amplifying circuit similar to that of Figure 4, but with the thyrlte" arranged to provide expansion' In this figure there is provided an inductor H9 in series with cathode I H of vacuum tube 95, and connected. to ground atv 2. Connected in"shunt across the inductor is a piece of "thyrite" H9. The induce tor builds up a voltage 180 degrees out of phase with the signal voltage being applied to grid 91, thereby opposing the input signal. voltage. The inductor introduces degeneration to this extent, and lowers the normal gain of the tube 95. The

ductor serves to limit the amount of voltage that can be present acrossinductor H9. By way of illustration, assume that this value is set at 5 volts. If the :thyrite were, not across the in-' ductor and the input voltage across terminals 90 and 9| were increased, the voltage across inductor 9 would increase in the same ratio, causing degeneration in the same ratio, merely resulting in linear amplification.

across terminals 99 and 9| is increased, the potential across inductor H9 is not increased, since the thyrite shunts this voltage, due to its property noted above.

value or reduced, and non-linear amplification of an expanding quality is obtained. The "thyrite also serves to aid this result,'not only by limiting the degenerative eflect of the inductor H9 but also by decreasing the bias in the cathode-grid causing the voltage between the output terminals 92 and 93 to remain approximately constant, regardless of variation in input signal over a wide range and above a threshold or predetermined value. By way of example, this may be explained as follows: Assume that the amplifier tube 95 has a gain of 10. The voltage between tenth to one volt, but the voltage across resistor 94 will remain approximately constant at one volt. The threshold level at which this circuit operates may be controlled by variableresistor lol in series with th "thyrite and condenser.

input terminals 99 and 9! may vary from one- It should be pointed out that the condenser 99 is not necessary in more than one stage of amcircuit. The threshold level at which expansion takes place may be controlled by variable resistor lit in series with the. "thyrite.

It will be understood that in lieu of "thyrite any of the negative resistance materials discussed, or any other circuit providing a resistance of such quality, may be used, thyrite being illustrative of a'preferred modification because of the variations in its qualities which can be obtained by different compositions and processes of manufacture. 7

It is to be further understood that the terms used and circuits illustrated are descriptive only, and that my invention includes such variations and equivalents as may be included within the scope of the appended claims. I

' Having thus fully described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In an amplifying circuit, a circuit having one end connected at a point of amplified voltage and the other end to a point of opposite voltage phase in the amplifying circuit, said circuit including a rectifier and a resistance means controlled thereby whose resistance progressively varies with a change in the voltage applied to it, said rectifier receiving energy directly from said amplifying circuit, said resistance means and rectifier being arranged in said circuit so as to vary the amount of voltage being applied by said circult to said point in response to a varying input signal, whereby a portion of the energy derived from the amplifier output circuit is rectified and fed to the resistance for controlling purposes.

2. In an amplifying circuit, a. circuit having one end connectedat a point of amplified voltage and the other end to a point of opposite plification, as the thyrite may be connected voltage phase in the amplifying circuit, saidcir- With the presence. of thyrite across inductor H9, as the voltage 1 In this way, the degenerative efiect of inductor H9 is held to a given cult including a grid controlled vacuum tube serving as a variable resistance, a second circuit including a rectifier receiving energy directly from said amplifying circuit and feeding rectified energy directly to the grid of said vacuum tube so as to vary the negative bias of said grid as the signal applied to said amplifier'varies, whereby a portion of the energy derived from the amplifier output circuit is rectified and fed to the vacuum tube for controlling purposes.

3. In an amplifying circuit, a circuit having one end connected at a point of amplified voltage and the other end to a point of opposite voltage phase in the amplifying circuit, said circuit including a rectifier and a resistance means controlled thereby, whose resistance progressively increases with an increase in the voltage applied to it, said rectifier receiving energy directly from said amplifying circuit, said resistance means and rectifier being arranged in said circuit so as to increase the amount of voltage being applied by said circuit to said point in response to an increase of the input signal, whereby a portion of the energy derived from the amplifier output circuit is rectified and fed to the resistance for controlling purposes.

4. In an amplifying circuit, a circuit having one end connected at a point of amplified voltage and the other end to a point of-opposite voltage phase in the amplifying circuit, said circuit including a rectifier and a resistance means controlled thereby whose resistance progressively decreases with a decrease in the voltage applied to it, said rectifier receiving energy directly from said amplifyin circuit, said resistance means and rectifier being arranged in said circuit so as to decrease the amount of voltage being applied by said circuit to said point in response to an increase of the input signal, whereby a portion of the energy derived from the amplifier output circuit is rectified and fed to the resistance for controlling purposes.

5. In an amplifying circuit, a circuit having one end connected at, a point of amplified voltage and the other end to a point of opposite voltage phase in the amplifying circuit, said circuit including a grid controlled vacuum tube serving as a variable resistance, a second circuit including a rectifier receiving energy directly from said amplifying circuit and feeding rectified energy directly to the grid of said vacuum tube so as to decrease the negative bias of said grid as the signal applied to said amplifier decreases, whereby a portion of the energy derived from the amplifier output circuit is rectified and fed to the vacuum tube for controlling purposes.

6. In an amplifying circuit, a circuit having one end connected at a point'of amplified voltage and the other end to a point of opposite volt-'- age phase in the amplifying circuit, said circuit' including a grid controlled vacuum tube serving as a variable resistance, a. second circuit including a rectifier receiving energy directly from said amplifying circuit and feeding rectified energy directly to the grid of said vacuum tube so as to increase the negative bias of said grid as the signal applied to said amplifier increases, whereby a portion of the energy derived from the amplifier output circuit is rectified and fed to the vacuum tube for controlling purposes.

EDWARD M. SORENSEN. 

